Not only have supporters become unaccustomed to seeing their side lose on the road under Ole Gunnar Solskjaer, but the manner of the performance was also quite odd.

It could be argued that United actually played better at the Emirates on Sunday afternoon than they had to defeat Paris Saint-Germain last week, however the juxtaposition in mood come full-time over the weekend was quite the contrast from the sheer disbelief of Wednesday night.

With some better finishing in front of goal and different officiating United might well have got more reward for their endeavours in North London, but it just wasn't to be.

However, there was one aspect of United's play which will strike warning into decision makers at the club ahead of their overhaul in the summer transfer window.

The centre-back partnership between Victor Lindelof and Chris Smalling has been one of the central pillars of the new foundations which have already been laid by Solskjaer since his appointment as caretaker manager. Perhaps that is why their uncharacteristic horror show will be of such alarm to fans who watched them both ran ragged by the clever attacking of Alexandre Lacazette and Pierre-Emerick Aubameyang.

It is no illusion that there have been warning signs in recent weeks regarding the vulnerability of United's back line, but the overall confidence in the team has often enabled Lindelof and Smalling to benefit from less scrutiny on their individual play.

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It should also be pointed out that much of United's improved defending has come when their first-choice defensive duo have been protected by a confident midfield trio of Nemanja Matic, Ander Herrera and Paul Pogba. The fact Herrera was missing, and the other two went missing is perhaps part of the reason behind Solskjaer's first domestic defeat since taking temporary charge.

Nonetheless it was alarming to see Arsenal's fluid front two establish such a foothold on proceedings and exploit a vulnerability which many fans had pushed to the backs of their minds.

United have known for well over a year now that they simply must reinforce their defensive options if they want to establish the same solidity which has transformed Liverpool into Premier League title challengers.

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Eric Bailly's disastrous display as an auxiliary right-back last week will have shattered his confidence and cast fresh doubts over whether or not United even have a suitable reserve centre-back to freshen things up when neccessary.

Marcos Rojo's perpetual fitness woes and the inconsistency of Phil Jones only further muddy the mix, while the lack of any quality youth options ready for a first-team birth make it essential for United to strengthen at the end of the season.

It's remarkable to think how far United have come in the last three months but if they want this upward trajectory to continue when they start the new season they will need a new top-class central defender in the squad.